The documentation says that "You can either contact the person who is adding the broken entry to a tag or just ignore it if such isn't majorly needed", but it never explains how I should find the offending mod or datapack. I've tried looking for a debug message of some sorts in both "logs/latest.log" and "logs/debug.log", but to no avail - the only thing I spot is this line:

[23nov.2024 15:40:21.887] [Render thread/INFO] [net.minecraft.client.gui.components.ChatComponent/]: [System] [CHAT] [Load My F***ing Tags]: It seems that some tags are a bit cooked. Look at the Logs for more details on broken functions. Click me for more info about this feature.

Can you please sort this out? How do I find the mod that causes the problem in the first place? Any help is appreciated. Thanks!

You need to check for tag-loading errors before such a message as mainly it will state on the server log that the tag has entries failing to load due to not existing.